The following invention relates to a control circuit for an electric motor and more particularly to a vector control circuit to control a DC brushless motor.
Existing sensorless six step motor drivers encounter severe starting problems because of harmonic noise present at the motor terminals caused by large torque variations in the excitation waveforms provided to the motor terminals from the motor driver. As an example, when the first pulse (after the homing pulse) is applied to the motor terminals using a six step motor driver, the torque on the motor steps from zero torque to 83 percent of the maximum torque. It is nearly impossible to accurately predict the position of the rotor in the motor when a second pulse is applied to the motor terminals. Without predictability of the rotor position the motor will almost inevitably experience a rough start. Motor driver designers have attempted to obtain a smoother motor start by employing additional damping resistors at the motor terminals, or using some form of mechanical damping applied to the rotor of the motor. Although such damping techniques result in an improvement in starting, the large torque variations are not eliminated, accordingly starting problems still persist. Additionally, the damping increases the time required for the rotor to reach full speed. Furthermore, turning on and off each phase six times for each revolution of the rotor introduces harmonic content into the interior motor waveforms which results in more frequencies which can in turn cause resonances resulting in unwanted acoustic noise.
In U.S. Pat. No. 5,034,668, assigned to the same assignee, the motor flux is determined dynamically by sensing the current and voltage at the motor terminals. The quadrature relation of flux and current is established by converting the three phase motor flux to a derived two phase motor flux, and employing a phase locked loop to generate the driving current from the derived motor flux. The two phase representation of the flux is converted to three phases to drive the motor. The use of the conversion matrices complicates and adds additional expense to the motor driver. To actually measure the motor flux, additional expense is incurred by requiring a flux sensor on the motor. Additionally, the value of the motor inductance and resistance has to be predetermined for each different motor type to allow component values in the driver circuit to be properly set. Without making a proper measurement of the particular motor's inductance and resistance, the measured flux will be in error causing operational problems. Further, the flux detector circuit employs an integrator circuit that tends to make inaccurate measurements at slow rotor speeds (during operation and startup), because of the required bleeder resistors across the integrating capacitor.
A typical motor speed control device allows for at most 24 speed updates of the rotor per revolution. This becomes an undesirable limiting bandwidth of the device. Plunkett, U.S. Pat. No. 4,928,043 discloses an example of such a device.
What is therefore desired is a motor driver that minimizes motor torque variations when starting and allows for a faster startup time. The bandwidth of the device should be increased substantially to allow for finer speed control. Additionally, during operation the interior motor waveforms should have minimal harmonic content to minimize acoustic noise.